c++ read binary data contained in an string variable - c++

Question

I have a function that uses zlib to unzip files from a zip file to memory, and then stores the resulting file content in a string object. The problem is that the string is now composed of the binary representation of the files, and I don't know how to read that string in blocks like with
ifstream::read():
file.read((char*)result, sizeof(int));
It's necessary that I read the data in block by block, but I haven't found any process to do that from a string variable.
This is the function that returns the file content as a string (I can change the function if there's a better approach):
string externalresourcemanager::getFileInsideZip(string zipFile, string fileInZip) {
int err = UNZ_OK; // error status
uInt size_buf = WRITEBUFFERSIZE; // byte size of buffer to store raw csv data
void* buf; // the buffer
string sout; // output strings
char filename_inzip[256]; // for unzGetCurrentFileInfo
unz_file_info file_info; // for unzGetCurrentFileInfo
unzFile uf = unzOpen(zipFile.c_str()); // open zipfile stream
if (uf==NULL) {
cerr << "Cannot open " << zipFile << endl;
return sout;
} // file is open
if ( unzLocateFile(uf,fileInZip.c_str(),1) ) { // try to locate file inside zip
// second argument of unzLocateFile: 1 = case sensitive, 0 = case-insensitive
cerr << "File " << fileInZip << " not found in " << zipFile << endl;
return sout;
} // file inside zip found
if (unzGetCurrentFileInfo(uf,&file_info,filename_inzip,sizeof(filename_inzip),NULL,0,NULL,0)) {
cerr << "Error " << err << " with zipfile " << zipFile << " in unzGetCurrentFileInfo." << endl;
return sout;
} // obtained the necessary details about file inside zip
buf = (void*)malloc(size_buf); // setup buffer
if (buf==NULL) {
cerr << "Error allocating memory for read buffer" << endl;
return sout;
} // buffer ready
err = unzOpenCurrentFilePassword(uf,NULL); // Open the file inside the zip (password = NULL)
if (err!=UNZ_OK) {
cerr << "Error " << err << " with zipfile " << zipFile << " in unzOpenCurrentFilePassword." << endl;
return sout;
} // file inside the zip is open
// Copy contents of the file inside the zip to the buffer
cout << "Extracting: " << filename_inzip << " from " << zipFile << endl;
do {
err = unzReadCurrentFile(uf,buf,size_buf);
if (err<0) {
cerr << "Error " << err << " with zipfile " << zipFile << " in unzReadCurrentFile" << endl;
sout = ""; // empty output string
break;
}
// copy the buffer to a string
if (err>0) for (int i = 0; i < (int) err; i++) sout.push_back( *(((char*)buf)+i) );
} while (err>0);
err = unzCloseCurrentFile (uf); // close the zipfile
if (err!=UNZ_OK) {
cerr << "Error " << err << " with zipfile " << zipFile << " in unzCloseCurrentFile" << endl;
sout = ""; // empty output string
}
free(buf); // free up buffer memory
return sout;
}
Thank you!
EDIT:
im trying your solution (john) but i get an empty result. Here is an example code to explain the problem:
int store = 1024;
string result = "";
ofstream file("test.txt", std::ios::binary);
//save a few data
file.write(reinterpret_cast<const char*>(&store), sizeof(int));
file.write(reinterpret_cast<const char*>(&store), sizeof(int));
file.close();
//remember that i receive the "string binary data" from the getFileInsideZip() function (here i simulating the problem and store the binary data in a string only for explanation purpose)
ifstream newFile("test.txt", std::ios::binary);
newFile.seekg(0, std::ios::end);
int length = newFile.tellg();
newFile.seekg(0, std::ios::beg);
char* begin = &*result.begin();
//here i have a binary file in a string variable
newFile.read(begin, length);
newFile.close();
cout << "result:" << result << "\n";
//now im trying to get the first int from this binary data
int stored = 0;
std::istringstream bin(result);
bin.read((char*)&stored, sizeof(int));
cout << "stored: " << stored << "\n";
//but the output is:
//result -> empty (initial value)
//stored: 0 -> (initial value)
any suggestion?